Here's some ideas (some from me, some from the internet:
Maybe you can try replacing the SI board. In my car, the batteries had leaked acid onto some of the wires and shorted them out. I was able to see this damage. If your temp gauage and or lights for the cluster are affected, this could be your problem.
Also, check to make sure that all the connections at the rear of the cluster are properly plugged in.
Check fuse #10.
The tach gets it's signal from the ECU behind the glove box.
The wiring at the connector plug for the speed sensor (in the differential housing) may be faulty and corroded …. pull the boot back from the plug housing and inspect the wires and the pins.